1. Field of the Disclosure
The present disclosure relates to a photo module which measures a signal strength in a transmission path, and this technique collects, on a light receiving element, signal light guided to a gradient index lens in the photo module and, at the same time, prevents reflected light returned from the gradient index lens in the photo module from entering into the transmission path again.
2. Discussion of the Background Art
There is a photo module which measures the signal strength in the transmission path and thereby monitors a communication state in the transmission path (for example, see Patent Literature 1).
A line monitor disclosed in Patent Literature 1 is constituted of an incident light fiber, an emitting light fiber, a rod lens, a reflection coating including a light transmitting portion, and a light receiving/detecting element. An optical signal guided from the transmission path reaches the reflection coating through the incident light fiber and the rod lens. The optical signal having reached the light transmitting portion of the reflection coating is detected by the light receiving/detecting element. The optical signal having reached the light transmitting portion of the reflection coating is output to the transmission path through the rod lens and the emitting light fiber.    Patent Literature 1: Japanese Patent Application Laid-Open No. 2-141709
As described above, in Patent Literature 1, with the provision of the emitting light fiber and the reflection coating, some of the optical signals guided from the transmission path are detected by the light receiving/detecting element, and the remaining portions of the optical signal are output to the transmission path. It is considered that all the optical signals guided from the transmission path are detected by the light receiving/detecting element, without providing the emitting light fiber and the reflection coating, in application of Patent Literature 1.
FIG. 1 shows a configuration of a photo module of a conventional technique. This photo module of the conventional technique is constituted of an incident light fiber 1, a gradient index lens 2, and a light receiving element 3. The incident light fiber 1 guides incident light. The gradient index lens 2 is joined to an exit light end of the incident light fiber 1, has an optical axis 21 different from the incident light fiber 1, has a ¼ period length with respect to the wavelength of the incident light, and has both end surfaces substantially vertical to the optical axis 21. The light receiving element 3 is disposed at an optical path position of the emitting light from the gradient index lens 2 and measures the strength of the emitting light from the gradient index lens 2.
An optical path of an optical signal in the photo module of the conventional technique will be described. The optical signal from the transmission path is guided on an optical path A in the incident light fiber 1 and reaches a point 22 deviated from the optical axis 21 in the gradient index lens 2. Since the period length of the gradient index lens 2 is a ¼ period length with respect to the wavelength of the incident light, the optical signal having reached the point 22 is guided on an optical path B and reaches a point 23 on the optical axis 21 in the gradient index lens 2. The optical signal having reached the point 23 is guided on the optical path C and reaches a point 31 in the light receiving element 3.
The optical path of the optical signal has some degree of spread. The optical signals having reached the point 22 are guided on optical paths B-1 and B-2 deviated by the spread from the optical path B and reach the respective points 25 and 26 deviated by the spread from the point 23 in the gradient index lens 2. Since the period length of the gradient index lens 2 is the ¼ period length with respect to the wavelength of the incident light, the optical signals having reached the points 25 and 26 are guided respectively on optical paths C-1 and C-2 parallel to an optical path C and reach points 32 and 33 deviated by the spread from the point 31 in the light receiving element 3. Accordingly, in the photo module of the conventional technique, the signal light is less likely to be collected by the light receiving element 3.
Some of the optical signals having reached the points 23, 25, and 26 are transmitted toward the light receiving element 3, and the remaining signals are reflected toward the incident light fiber 1. Since the period length of the gradient index lens 2 is the ¼ period length with respect to the wavelength of the incident light, the optical signals reflected by the points 23, 25, and 26 are guided respectively on optical paths D, D-1, and D-2 and collected at the point 24 in the gradient index lens 2. The points 22 and 24 are placed at symmetrical positions with respect to the optical axis 21. The optical signals reflected by the point 24 are guided on optical paths D, D-1, and D-2 and reach the points 23, 25, and 26 in the gradient index lens 2. The optical signals reflected by the points 23, 25, and 26 are guided on the optical paths B, B-1, and B-2 and collected at the point 22 in the gradient index lens 2. Accordingly, in the photo module of the conventional technique, the optical signal easily enters the incident light fiber 1 again because of multiple reflections in the gradient index lens 2.
In order to prevent the optical signal from being made enter the incident light fiber 1 again by being reflected at the points 22 and 24 and the points 23, 25, and 26, it is considered that end surface on the side of the points 22 and 24 and the end surface on the side of the points 23, 25, and 26 of the gradient index lens 2 are subjected to not substantially right angle polishing but bevel polishing, and an antireflection coating is provided on a substantially right angle polishing surface. However, application of the bevel polishing and provision of the antireflection coating increase the number of the manufacturing processes and the cost.
In order to solve the above problem, the present disclosure provides a photo module which measures a signal strength in a transmission path, and this technique collects a signal light, guided to a gradient index lens, on a light receiving element without increasing the number of manufacturing processes and the cost and, at the same time, prevents reflected light returned from the gradient index lens from entering inside the transmission path again.